The Everest lake was formed in the last few years because of glacial melting.

If drastic action is not taken Lewis is convinced nations will find themselves at war over water shortages.

In 2007 the WWF ambassador became the first person to swim across the North Pole.

Recalling that exploit, he said: “I remember standing on the edge of the ice about to dive into the water and thinking to myself, ‘I’ve never seen any place on this earth which is just so frightening.’

“The water is completely black. The water is minus 1.7ºC or 29ºF. It’s flipping freezing in that water.

“Then a thought came across my mind: ‘If things go pear shaped on this swim, how long will it take for my frozen body to sink the four and a half kilometres to the bottom of the ocean?’

“Then I said to myself, ‘I have got to get this thought out of my mind as quickly as possible.’”

He put his i-Pod on and psyched himself up listening to opera and Puff Daddy.

He dived in and 18 minutes, 50 seconds later he pulled himself from the water. It felt like 18 days.

“I remember getting out of the water and my hands feeling so painful,” he said. “I looked down at my fingers and they were literally the size of sausages.

“We are made partially of water and when water freezes it expands so the cells in my fingers had frozen, expanded and burst.”

Lewis nearly lost both hands as a result. He couldn’t feel them for four months after.

On undertaking the second Everest swim he knew if he swam too slowly he could face losing his hands again.

Dying was also a very real prospect.

But belief in himself and his mission drove him on.

Lewis was born in Plymouth and speaks with a thick South African accent after 15 years in the country, but considers “every drop” of himself to be Welsh.

He said: “Climate change is the Mount Everest of all problems.

“Just because we have lived the way we have for so long, and populated the earth the way we have for so long does not mean we can carry on the way we are carrying on. The warning signs are all there.”

Lewis, who did the Everest swim in May, is now planning his next feat from his South African base in Capetown.

He is cagey with the details. He’s not even sure if what he wants to do is even possible.

But he reveals if it happens, what he wants to do will be in the southern hemisphere and will take 120 days to finish.

He said: “It’s going to take place in two years’ time. It is such a monumental swim, I think it will take me 120 days to get from A to B.

“I started the training a few weeks back.”

Most days the qualified maritime lawyer walks to the beach near his home before running 10km. Then he swims for an hour before delivering two speeches.

When in Wales he trains at Llyn Llydaw on the flanks of Snowdon.

Last year he taught actor Robson Green how to swim across it for Robson Green’s Wild Swimming Adventure.

Lewis added: “As someone who has seen conflict, I will do virtually everything to avoid it.

“This (on Everest) was a swim for peace. It was the first of a number of swims for peace that I will be doing.”